The personal computer industry has three basic graphics display standards. The "monochrome" display standard allows crisp text on a monochrome display using a 720.times.350 format, or 720 dots or pixels in each horizontal line and 350 horizontal lines on the display. The "color graphics adapter" (CGA) standard supports 16 different colors on a display using a 640.times.200 format. The 640.times.200 format is adequate for graphics and supports text modes, but the text is so coarse that it is not very useful. A third standard is known as the "enhanced graphics adapter" (EGA). The EGA has monochrome modes (720.times.350 text and 640.times.350 graphics) and color modes (640.times.350 text and graphics as well as 640.times.200 text and graphics). The multiple modes allow the EGA to drive the original monochrome monitor, the original color monitor, and the new enhanced color display having both 640.times.200 and 640.times.350 modes.
A dual scan monitor (DSM) has been developed which has a low scan mode which is the same as the industry standard color monitor and a high scan mode which is unique to a particular application. The low scan mode has a horizontal scan frequency of 15.7 KHz and a 60 Hz frame rate. The high scan mode has a horizontal scan rate of 18.5 KHz and a 50 Hz frame rate.
The horizontal and vertical scan rates in the 640.times.200 resolution mode of the enhanced color display, the standard color monitor, and the dual scan monitor are the same. As a result, a display controller can drive an enhanced color display and a dual scan monitor or a standard color monitor and a dual scan monitor in the 640.times.200 resolution mode at the same time.
However, the horizontal and vertical scan rates in the 640.times.350 resolution mode of the enhanced color display and the dual scan monitor are not the same. As a result, a single display controller cannot drive both an enhanced color display and a dual scan monitor in the 640.times.350 resolution mode at the same time.